The efficient introduction of fluorine atoms into bioactive organic molecules has attracted considerable attention in recent years owing to the unique properties of the fluorine substituent. (Refs. 1-4) The selective replacement of hydrogen or oxygen with fluorine can change a compound's biological activity, metabolic stability, chemical stability, lipophilicity, acidity and dipole properties with modest change in steric bulk. (Refs 2-4) A broad class of particularly relevant compounds are the fluorinated carbohydrates (Refs. 1, 5, 6) and nucleosides. (Refs. 1, 3, 4) Selective fluorination in the sugar moiety of glycosides or nucleosides has proven useful to numerous investigations of enzyme mechanism where either sugars or nucleosides are substrates. (Refs. 6-14) Some of these compounds are potent drugs. For example, gemcitabine (Gemzar, Eli Lilly, 2′-deoxy-2′,2′-difluorocytidine) is used clinically to treat numerous cancers including ovarian (Ref. 15), pancreatic (Ref. 16), and breast (Ref. 17) cancers, with sales in excess of 1.6 billion dollars per year (Ref. 18). Of the mono-substituted 2′-fluoro-nucleosides, clofarabine (Clorar, Genzyme, 2-Chloro-2′-deoxy-2′-fluoro-arabino-adenosine), has been approved for pediatric patients with relapsed or refractory acute lymphocytic leukemia (Ref. 19) and annual sales now exceed 100 million dollars (Ref. 20).
Previous studies have demonstrated that 2′-deoxy-2′-fluoro-arabino-nicotinamide-mononucleotide is a potent mechanism-based inhibitor (apparent Ki=61 nM) of the signaling enzyme cell developmental protein 38 (CD38) (Refs. 10, 12). The 2′-fluoro-NAD+s and related compounds are likely to be valuable for the study of sirtuin enzyme mechanism (Refs. 11, 21), and for studying the chemical properties of poly-ADP-ribosylpolymerases (Ref. 22). Fluorinated NAD+s could be useful for identifying ADP-ribosyltransfer sites on proteins as well. For example, it has been demonstrated that the 2′-deoxy-2′-fluoro-arabino-furanosyl modification is suitably robust for MS/MS approaches used to characterize amino acid post-translational modifications (Ref. 10). ADP-ribosylation sites are poorly surveyed within the proteome and yet these modifications are of heightened interest as they are implicated in important biological effects (Refs. 22-24). The synthesis of 2′-deoxy-2′-fluoro-arabino-NAD+ was previously described (Ref. 25), but syntheses of 2′-deoxy-2′-fluoro-ribo-NAD+ and 2′-deoxy-2′,2′-difluoro-NAD+ or their nucleoside precursors have not appeared in the literature. The difluoro derivatives in particular are useful, based upon their chemical stability and altered electronic properties.
Methods to synthesize the 2-deoxy-2-fluoro-D-furanose precursors to 2′-fluorinated nucleosides have not experienced substantial recent innovation (Refs. 1, 3, 26) despite the fact that unmet need for these compounds has only increased in recent years. The synthetic methods currently available vary in efficiency and all depend on routes relying on different initial precursors to the respective final products. For example, the best method to make a protected 2-deoxy-2-fluoro-arabino-furanose requires only 2 steps from a protected ribose in 58% overall yield (Refs. 25, 27, 28, 29). On the other hand, 2-deoxy-2-fluoro-ribofuranose has no concise or efficient synthesis (Ref. 30) and still requires 6 steps from arabinose (Ref. 31). Alternatively, 2-deoxy-2-fluoro-ribofuranose can be obtained via a 10-step non-diastereoselective method in 11% overall yield (Refs. 32-34). An efficient but non-diastereoselective route has been established for the synthesis of protected 2-deoxy-2,2-difluoro ribonofuranose in 5 steps via coupling of ethyl-bromodifluoroacetate and isopropylidene glyceraldehyde. The route is not diastereoselective and depends upon crystallization of the preferred isomer (Ref. 35). This procedure was developed by Eli Lilly for commercial synthesis of gemcitabine. 2-Deoxy-2,2-difluororibofuranose can also be obtained stereoselectively from glucose or mannose (Ref 36), but that method involves 8 steps and very low overall yield (<15%).
In view of the foregoing, there is a desire to provide novel halogenated 2-deoxy nucleosides and method for the synthesis thereof.